xfs: Replace macro XFS_DQUOT_TREE with a function

In preparation for combined pquota/gquota support, for the sake
of readability, change the macro to an inline function.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

diff --cc fs/xfs/xfs_dquot.c
Simple merge

diff --cc fs/xfs/xfs_qm.c
Simple merge

diff --cc fs/xfs/xfs_qm.h
Simple merge